Thermionic cathode heated by electron bombardment

ABSTRACT

A structure wherein a cathode, which is made of a material, such as lanthanum hexa-borides, prone to react with metals at high temperatures and having a high electron emissivity, is held by a supporter which is made of an electrically insulating material. Concentric metal cylinders are arranged at the outer circumference of the supporter to surround a part of the cathode and a heating coil is arranged in the interstice between the metal cylinders. An electron-emissive metal oxide layer is formed on the inside surface of the inner metal cylinder. Thermions created from the oxide layer strike the cathode, and the cathode is heated by the heat of the electron bombardment.

United States Patent 1 Hosoki et al.

[ Dec. 23, 1975 THERMIONIC CATHODE HEATED BY ELECTRON BOMBARDMENT [75]Inventors: Shigeru Hosoki; Michio Ohtsuka,

both of Hachioji; Satoru Fukuhara, Kokubunji, all of Japan [73]Assignee: Hitachi, Ltd., Japan 7 [22] Filed: Dec. 10, 1973 [21] Appl.N0.: 423,107

[30] I Foreign Application Priority Data Dec. 8, 1972 Japan 47-122502[52] US. Cl. 313/270; 313/305; 313/339; 313/346 [51] Int. Cl. H01J 1/94;HOlJ 1/20 [58] Field of Search 313/337, 340, 346, 339, 313/46, 305, 338,347, 270, 90, 464

[56] References Cited UNITED STATES PATENTS 1,210,678 1/1917 Nicolson313/305 2,159,824 5/1939 Spanner 313/339 2,386,790 10/1945 Gaun et a1.313/270 2,561,768 7/1951 Adler 313/305 2,585,582 2/1952 Pierce 313/3373,333,138 7/1967 Szegho 313/270 3,369,145 2/1968 Domotor 313/337 OTHERPUBLICATIONS Lafi'erty, J. M., Boride Cathodes, Jr. of Applied Physics,Vol. 27, 3-1951, pp. 299-309.

Primary Examiner-Alfred E. Smith Assistant Examiner-Wm. H. PunterAttorney, Agent, or FirmCraig & Antonelli [57] ABSTRACT A structurewherein a cathode, which is made of a material, such as lanthanumhexa-borides, prone to react with metals at high temperatures and havinga high electron emissivity, is held by a supporter which is made of anelectrically insulating material. Concentric metal cylinders arearranged at the outer circumference of the supporter to surround a partof the cathode and a heating coil is arranged in the interstice betweenthe metal cylinders. An electron-emissive metal oxide layer is formed onthe inside surface of the inner metal cylinderf' Thermions created fromthe oxide layer strike the-cathode, and the cathode is heated by theheat of the-electron bombardment.

13 Claims, 12 Drawing Figures ues s w lOb THERMIONICCATHODE HEATED BYELECTRON 'BOMBARDMENT BACKGROUND ot; THE INVENTION rial, such aslanthanum hexa-borides (LaB and yttrium hexa-borides (YB prone toreactwith metals at high temperatures.

Description of the Prior Art In general, borides such as lanthanumhexa-borides (LaB and yttrium hexa-borides (YB have a small workfunction, and are suitable as cathode materials. However, because theyare liable to react with metals at high temperatures, it has beendifficult to heat these borides to working temperatures of approximatelyI ,300l ,8()OC. During the heating of the cathode, the direct heatingtype requires high power. Indirect heating type is, therefore, effectivein reducing power comsumption to as low a value as possible.

FIG. 1 is a schematic diagram showing an example of a prior-art cathodeheating device, which is constructed such that a heating coil 2, made oftungsten or the like, is held in a space surrounding cathode 1, made oflanthanum hexa-boride (LaB and heating power is supplied from a powersource 3 to the coil 2. Between the cathode l and the heating coil 2, anaccelerating power source 4 is connected for electron bombardment.

With such a construction, when the coil 2 is heated, the cathode l isheated by the radiant heat. Simultaneously therewith, thermions emittedfrom the coil 2 are drawn to the cathode l by the voltage of theaccelerating power source 4, and the cathode l is heated by heat whichis generated by the electron bombardment. In order to increase thetemperature .of the cathode l to working temperature the heating coil 2must be heated to a temperature of at least about 2,5 00-2,800C whenemploying a tungsten wire. However, when the coil 2 is heated to theabove-mentioned temperature, heat losses due to thermal conduction fromthe leads at both ends of the coil 2 to the'exterior and the heat lossdue to the thermal radiation from the. coil 2 become so great as not tobe negligible.

FIG. 2 shows a sketch for roughly estimating the heat losses due tothermal conduction and due to thermal radiation. Referring to thefigure, leads 5 of stainless steel are connected to both ends of theheating coil 2 of tungsten'in order to diminish the loss due to thethermal conduction. Now, let T denote the temperature of the centralpart of the heating coil 2 which is uniform, T denote the temperature ofthe point of contact between the coil 2 and the lead 5, and T denote.the temperature of the end of the lead 5 remote from the coil 2.lt isassumed that the diameter ofthe coil 2 is 0.02 cm, that the length ofthe coil extended in a straight line is 2.0 cm, that the sectional areaof the lead 5 is 4.45 X cm, that T 2,800C, that T I,500C and that T,l,000 CI When, although not shown in the figure, a heat shielding plateis provided around the coil 2, the temperature of the heat shieldingplate is assumed to be I,Q00C. Then, the radiation heat loss and thecondition heat loss can be respectively calculated to'be approximatelytO W and W.

In this manner, where the coil 2 is heated to a high temperature, theheat loss due to thermal radiation becomes very large, since it isproportional to the fourth power of the temperature in accordance withBoltzmanns law. When mounting a plurality of heat shielding plates inthe space surrounding the coil 2 in the form of concentric cylinders,the radiation heat loss can be reduced. In this case, however, the heatcapacity of the shielding plate assembly itself becomes large, and thetime required for increasing the cathode temperature becomes large,resulting in the disadvantage that the heating device is very difficultto use.

SUMMARY OF THE INVENTION An object of the present invention is toprovide a device for heating a cathode made of a material such aslanthanum hexa-borides (LaB in which the heat loss by the thermalradiation is small and the heating efficiency is high.

In order to accomplish this object, the present invention includes aheating coil provided within an indirect heating case and anelectron-emissive metal oxide layer formed on the surface of theindirect heating case facing to a cathode.

The other objects and features of the present invention will be apparentfrom the following detailed description when read in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view depicting aprior-art heating arrangement;

FIG. 2 is a schematic view for calculating losses due to thermalradiation and thermal conduction froma heating coil; I

FIG. 3 is a constructional view of an embodiment of the presentinvention;

FIG. 4 is a schematic view for calculating thermal conduction;

FIG. 5 is a structural view of a cathode;

FIG. 6 is a constructional view showing another embodiment of thepresent invention;

FIG. 7 is a sectional view of a portion of still another embodiment ofthe present invention;

FIGS. 8a to 8d are sectional views of supporters for use in the presentinvention; and

FIG. 9 is a structural view in the case where the heating device of thepresent invention is applied to an actual cathode.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 3, a cathode 1made of a boride material such as lanthanum hexa-boride (LaB and yttriumhexa-boride (YB is positioned in a hollow core portion of a supporter 6made of a high temperature-resistant material, such as boron nitride(BN), which is electrically insulating. Outside the supporter 6,

. there are arranged concentric cylinder bodies 10a and 10b made ofnickel or the like. The end parts of the cylinders on one side areconnected by a metal sheet 100. Formed on the interior surface of thecylinder 10b is an electron-emissive wall made of a coating 9 which ismade of an electron-emissive metal oxide such as barium oxide (BaO),calcium oxide (CaO) and strontium oxide (SrO). An electrode 7 ofgraphite or the like is mounted on one end of the cathode l, and isconnected through a lead 8 to the positive terminal of an electronaccelerating power source 4 for electron bombardment. The negativeterminal of the power source 4 is connected to the cylinder a. A heatingcoil 2 of tungsten or the like insulated by an alumina coating layer isarranged between the concentric cylinders 10a and 10b, and is suppliedwith heating power from a power source 3.

With such a construction, when current flows through the coil 2 by meansof the heating power source 3 and the coil 2 is thus heated, thecylinders 10a and 10b are also heated. As a consequence, the oxide.layer 9 is subjected to the indirect heating. When the oxide layer 9 isheated to approximately 800C, thermions are emitted. Since the cathode lis applied with a positive potential through the electrode 7, which ismade of graphite or the like material and does not readily react withthe cathode at high temperatures the thermions emitted from the oxidelayer 9 are attracted toward the cathode l and impinge thereon. Thecathode l is then heated to approximately l,300-I,800C by the heat ofthe electron bombardment. Since, in this case, the cylinder 10b has atemperature lower than the cathode 1, heat losses due to thermalconduction and thermal radiation from the cathode l are mostly fedbackto the cylinder 10b as is apparent from the construction shown in thefigure. Accordingly, once the cylinders 10a and 10b have been heated,the power required for maintaining them at 800C may be very slight. Heatlosses due to thermal radiation from the cylinder 10a are extremelysmall, because the temperature of this cylinder is as low as 800C.

As explained above, according to the cathode heating device of thepresent invention, the cathode 1 is not heated directly by the radiantheat of the heating coil 2, but is heated by the electron bombardmentheat in such way that the metal cylinders 10a and 10b are heated andthat thermions emitted from the oxide layer 9 formed on the insidesurface of the cylinder 10b are accelerated and impinge upon the cathode1, so that the cathode 1 can be heated to a desired high temperature inthe state in which the metal cylinders 10a and 1012 are at a temperaturelower than that of the cathode 1. From the heating by the electronbombardment heat, the heat loss components are those due to thermalconduction from the cathode 1 through the supporter 6 to the cylinder10b and these directly escaping due to thermal radiation from thecathode 1. Since both these heat loss components are fed-back to thecylinder 10b surrounding the cathode, there is essentially no heatloss,and a highly efficient heating can be effected.

In the foregoing embodiment, the material of the cylinders 10a and 10bis not restricted to nickel, but may be any metal having deoxidizingproperties at a high temperature of about 800C. The coating layer is notrestricted to electron-emissive metal oxide layer 9 but a layer of asintered body or an impregnation layer of a porous metal may be adopted,insofar as it has an electron emissivity equivalent to that of thecoating layer 9.

Although the disadvantages of the prior-art device have been eliminatedby the device of the embodiment shown in FIG. 3, even the improveddevice has a few problems. One of them is that as the loss heat from thecathode I is effectively fed-back to the cylinder 10b, the temperatureof the metal oxide layer 9 increases more than is necessary, with theresult that the deterioration of the oxide layer 9 is hastened.

The excessive increase in the temperature of the oxide layer 9 in FIG. 3is attributable to the fact that the thermal feedback from the cathode Iis too great. As already explained, the causes for the thermal feedbackare l the thermal radiation from the cathode l and 2 the thermalconduction through the supporter 6. Rough estimates will be hereunderexplained for both mechanisms of thermal feedback.

The desired temperatures are approximately l,500C for the cathode I madeof LaB or the like and approximately 800C for the oxide layer 9. For thesake of simplicity, therefore, those quantities of heat flow, for therespective heat transfer mechanisms when the temperature differencebetween the specified values is assumed may be determined.

FIG. 4 indicates the dimensions necessary for such calculations. Let Orbe the quantity of heat which is imparted from the cathode l to theoxide layer 9 (the temperature of which is equal to that of the cylinder10b) by thermal radiation, and Qc be the quantity of heat which isimparted through the supporter 6 by thermal conduction. T and T are thetemperatures of the cathode l and the oxide layer 9 respectively. 6 isthe emissivity, 0' is Stephen-Boltzmanns constant, and K the coefficientof heat transfer of the supporter 6. Then, the equations of heattransfer due to thermal conduction and radiation are respectively givenas follows:

Qr= e 017 T 1r d I, 2. NOW let T10: l,500C, T20 800C, F 0.5 and 0' 5.67X I0 Joule lsec cm K When employing boron nitride for the supporter 6,the coefficient of heat transfer becomes K 0.63 Joule/cm-sec-C. Thedimensions in FIG. 4 are d, 0.1 cm, d 0.4 cm, I

0.2 cm and 1 0.3 cm. Then, there are obtained:

Qc 4.0 x 10 (Watt) 3. Qr= 1.99 (Watt) 4.

'I t is understood that for the dimensions d d l and I given above,thermal conduction through the supporter. 6 is very great when usingboron nitride.

When the temperature difference between the cathode I and the oxidelayer 9 is to be kept at about 700C, a calculated value of powercorresponding to the heat loss component (about 400 Watts) must beapplied to the cathode 1. In actuality, thermal contact resistancesexist between the cathode l and the supporter 6 and between thesupporter 6 and the cylinder 10b, 'iespectively, and the coefficient ofheat transfer K appaifently becomes smaller by one order or so. Thepower to be applied to the cathode 1 is, actually, less than 10 Watts.Under this condition, the temperature difference of 700C is notestablished, and the tempe rature of the oxide layer 9 of approximately1,300C has been observed when the temperature of the cathode 1 is l,500C. Consequently, if a temperature difference of 700C is to bemaintained without changing the dimensions, an insulating materialhaving a smaller coeff cient of heat transfer K by approximately oneorder must be employed. According to calculations, the heat loss isabout 400 W when employing boron nitride for the supporter 6, and hence,the coefficient of heat transfer must be decreased by two orders forrestraining the loss to less than 10 Watts. As previously explained,however, thermal contact resistances exist between the adjacent ones ofthe cathodel supporter 6- cylinder b, and hence, the decrease of thecoefficient of heat transfer by approximately one order suffices. lnthis regard, alumina'has a coefficient of heat transfer of;I( 0.07-Joule/ sec cm C (at 800C), which is about one order smaller than thecoefficient of heat transfer of boron nitride With a cathode of. theabove dimensions, therefore, the use of alumina is preferable to boronnitride. According to experiments, the value 700C has been observed asthe temperature difference between the cathode 1 and the oxide layer 9,and the life of the oxide layer 9 has been extended.

Where the supporter 6 is specified beforehand, the desired temperaturedifference can be established by appropriately determining thedimensions 11,, d 1 and l or by making the areas and shapes of thecontact surfaces between the cathode 1 and the supporter 6 and betweenthe supporter 6 and the cylinder 10b different.

The power consumption of the cathode can thus be minimized by selectingthe insulating material of the supporter 6 in dependence on the shapeand dimensions of the cathode l with reference to equations (1) and (2)for Qc and Qr respectively.

Where the cathode 1 is heated to a temperature (for example, 2000C)considerably higher than the usual working temperature, the temperatureof the contact part between the supporter 6 and the cathode l increases,and both members chemically react with each other in some cases. In thatevent, a layer 11 made of graphite powder or a sintered body thereof maybe formed between the cathode 1 and the supporter 6, as shown in FIG. 5.

Another problem of the embodiment in FIG..3 is that when the cathode lis used at high temperatures for a long period of time, a thin filmwhich is electrically conductive is formed on the surface of thesupporter 6 of the high temperature-resistant insulating material by thevaporization of the cathode material such as LaB resulting in adegradation of the insulation between the cathodel and the oxide layer9.

In order to solve such a problem, an evaporation preventing plate 12 maybe provided in proximity to the supporter 6 as illustrated in FIG. 6.Since the evaporation preventing plate 12 functions as a mask for thevaporization of the cathode material and prevents the material fromadhering to the surface of the supporter 6, there is good electricalinsulation between the cathode 1 and the oxide layer 9 for a long periodof time.

Even when a supporter 6' of a shape as shown in FIG. 7 is employedinstead of the provision of the preventing plate 12, the same effect isachieved. An part 6a of the supporter 6' becomes the so-called shadeportion with respect to the cathode 1, and the cathode material such asLaB cannot be readily deposited on the portion. Even when supporters 6of shapes illustrated in FIGS. 8a to 8d are used, the same effect isachieved. Any of the shapes has an extended portion which defines ashade with respect to the cathode l and on which LaB or the like cannotbe readily deposited," so that there is good electrical insulationbetween the cathode l and the oxide layer 9 is even during the use ofthe cathode for a long period of time.

FIG. 9 shows an overall concrete structure of the heating device of thepresent invention. In the figure, the same constituent parts as in H0. 3are assigned with the same symbols. The aperture of the metal sheet forconnecting the end parts of the cylindrical bodies 10a and 10b ismade-smaller than the inside diameter of the cylindrical oxide layer 9.This serves to prevent thermions for the electron bombardment, emittedfrom the oxide layer 9, from being mixed into the thermions which areemitted from the cathode 1 towards the opening portion of a Wehneltelectrode 16 or a grid electrode. The heating coil 2 has power suppliedthereto through lead wires from a power source (not shown) which isdisposed outside a cathode base 14 of glass or the like. The cylinder10a is connected to electrode terminals by lead wires 13.

While we have shown and described several embodi ments in accordancewith the present invention, it is understood that the same is notlimited thereto but is susceptible of numerous changes and modificationsas known to those skilled in the art and we, therefore, do not wish tobe limited to the details shown and described herein but intend to coverall such changes and modifications as are encompassed by the scope ofthe appended claims.

We claim:

1. A thermionic cathode structure comprising:

a cathode made of a material having a high electron emissivity;

a supporter made of an electrically-insulating and hightemperature-resistant material for supporting a part of said cathode;

a first cylinder held in contact with said supporter and arranged so asto surround a part of said cathode;

a second cylinder arranged concentrically with said first cylinder;

a cathode heating coil arranged in an interstitial space between saidfirst and second cylinders;

an electron-emissive metal oxide layer formed on an inside surface ofsaid first cylinder;

flange means, extending from said first cylinder toward said cathodepast said oxide layer, for preventing electrons emitted from said oxidelayer from being mixed with electrons emitted from a part of saidcathode;

a power source connected to said coil for supplying heating power tosaid coil; and

a power source connected between said cathode and said first cylinder inorder to cause electrons, emit ted from said oxide layer, to strikeagainst said cathode.

2. The structure according to claim 1, wherein said cathode is made of aboride.

3. The structure according to claim 1, wherein said oxide layer is madeof at least one member selected from the group consisting of bariumoxide, strontium oxide and calcium oxide.

4. in a thermionic cathode structure having a cathode;

an insulating body for insulatingly supporting said cathode; and

a heating coil, disposed adjacent said cathode;

the improvement comprising:

means, disposed between said heating coil and said cathode andsurrounding at least part-of said cathode, for emitting thermions whichimpinge upon said cathode to heat said cathode, in response to theheating of said heating coil, wherein said means comprises a firstcylinder supported by said insulating body and surrounding said cathodeand a layer of electron-emissive material coated on the interior surfaceof said fist cylinder facing said cathode; and

a flange extending from said first cylinder toward said cathode pastsaid layer of electron-emissive material, said electron-emissive layerbeing confined between said flange and said insulating body.

5. The improvement according to claim 4, wherein said means comprises awall made of an electron-emissive material which, when heated by saidcoil, emits thermions which impinge upon said cathode to heat saidcathode.

6. The improvement according to claim 4, wherein said means furtherincludes a second cylinder surrounding said coil, with said coildisposed between said second cylinder and said first cylinder.

7. The improvement according to claim 6, wherein said means furtherincludes a first power source for applying a potential between saidfirst cylinder and said cathode and a second power source for applying aheating current to said heating coil.

8. The improvement according to claim 4, wherein said layer ofelectron-emissive material is made of a 8 material selected from thegroup consisting of a metal oxide, a sintered body, and a porousmetal-impregnated layer.

9. The improvement according to claim 8, wherein said metal oxide ismade of at least one material selected from the group consisting ofbarium oxide, strontium oxide and calcium oxide.

10. The improvement according to claim 4, wherein said cathode is madeof a boride.

11. The improvement according to claim 4, further including a layer ofgraphite disposed between said insulating body and said cathode.

12. The improvement according to claim 4, further including anevaporation preventing plate disposed between said insulating body andsaid electron-emissive wall.

13. The improvement according to claim 4, wherein said insulating bodyhas an extended portion defining a shade area between said wall and saidcathode to prevent the formation of an electrically conductive filmbetween said wall and said cathode.

* II! II

1. A thermionic cathode structure comprising: a cathode made of amaterial having a high electron emissivity; a supporter made of anelectrically-insulating and high temperature-resistant material forsupporting a part of said cathode; a first cylinder held in contact withsaid supporter and arranged so as to surround a part of said cathode; asecond cylinder arranged concentrically with said first cylinder; acathode heating coil arranged in an interstitial space between saidfirst and second cylinders; an electron-emissive metal oxide layerformed on an inside surface of said first cylinder; flange means,extending from said first cylinder toward said cathode past said oxidelayer, for preventing electrons emitted from said oxide layer from beingmixed with electrons emitted from a part of said cathode; a power sourceconnected to said coil for supplying heating power to said coil; and apower source connected between said cathode and said first cylinder inorder to cause electrons, emitted from said oxide layer, to strikeagainst said cathode.
 2. The structure according to claim 1, whereinsaid cathode is made of a boride.
 3. The structure according to claim 1,wherein said oxide layer is made of at least one member selected fromthe group consisting of barium oxide, strontium oxide and calcium oxide.4. In a thermionic cathode structure having a cathode; an insulatingbody for insulatingly supporting said cathode; and a heating coil,disposed adjacent said cathode; the improvement comprising: means,disposed between said heating coil and said cathode and surrounding atleast part of said cathode, for emitting thermions which impinge uponsaid cathode to heat said cathode, in response to the heating of saidheating coil, wherein said means comprises a first cylinder supported bysaid insulating body and surrounding said cathode and a layer ofelectron-emissive material coated on the interior surface of said fistcylinder facing said cathode; and a flange extending from said firstcylinder toward said cathode past said layer of electron-emissivematerial, said electron-emissive layer being confined between saidflange and said insulating body.
 5. The improvement according to claim4, wherein said means comprises a wall made of an electron-emissivematerial which, when heated by said coil, emits thermions which impingeupon said cathode to heat said cathode.
 6. The improvement according toclaim 4, wherein said means further includes a second cylindersurrounding said coil, with said coil disposed between said secondcylinder and said first cylinder.
 7. The improvement according to claim6, wherein said means further includes a first power source for applyinga potential between said first cylinder and said cathode and a secondpower source for applying a heating current to said heating coil.
 8. Theimprovement according to claim 4, wherein said layer ofelectron-emissive material is made of a material selected from the groupconsisting of a metal oxide, a sintered body, and a porousmetal-impregnated layer.
 9. The improvement according to claim 8,wherein said metal oxide is made of at least one material selected fromthe group consisting of barium oxide, strontium oxiDe and calcium oxide.10. The improvement according to claim 4, wherein said cathode is madeof a boride.
 11. The improvement according to claim 4, further includinga layer of graphite disposed between said insulating body and saidcathode.
 12. The improvement according to claim 4, further including anevaporation preventing plate disposed between said insulating body andsaid electron-emissive wall.
 13. The improvement according to claim 4,wherein said insulating body has an extended portion defining a shadearea between said wall and said cathode to prevent the formation of anelectrically conductive film between said wall and said cathode.